Bipolar junction transistors are commonly employed in integrated circuits that require high-speed amplifiers or high-speed switches. A bipolar junction transistor (BJT) comprises three adjacent doped semiconductor regions having an NPN or PNP doping configuration. A middle region forms a base and two end regions separated by the base form an emitter and a collector. The middle base region is physically narrow relative to the minority carrier diffusion length for carriers within the base. Typically, the emitter has a higher dopant concentration than the base and the collector, and the base has a higher dopant concentration than the collector. A small signal applied to one of the BJT terminals modulates large changes in current through the other two terminals. A BJT operates to amplify an input signal supplied between the base and the emitter, with the output signal appearing across the emitter/collector. The BJT can also operate as a switch with an input signal applied across the base/emitter junction switching the emitter/collector circuit to an opened or a closed (i.e., short-circuited) state.
The emitter current primarily comprises the injection of carriers from the emitter into the base, which is achieved by making the donor concentration in the emitter much greater than the acceptor concentration in the base. Thus for the common NPN BJT, electrons are injected into the base with negligible hole injection into the emitter from the base. Since the base is very narrow compared to the minority carrier diffusion length (the diffusion length of the electrons in the base), the carriers injected into the base do not recombine in the base, but diffuse across the base into the reverse-biased base-collector junction. Thus a current across the reverse-biased base-collector junction is determined by the carriers injected from the emitter that arrive at the base-collector depletion region. The dopant concentration in the collector is less than that in the base, so the depletion region extends primarily into the collector.
There are several known semiconductor fabrication processes for forming the three doped regions of a bipolar junction transistor, and several different BJT architectures can be formed according to these processes. The simplest structure comprises a planar architecture having stacked NPN or PNP regions formed by successive dopant implants into a silicon substrate.
Significant performance enhancements are achieved by a heterojunction bipolar junction transistor (HBT) having a silicon-germanium base. It is known that the silicon-germanium base exhibits a narrower band gap and lower resistivity than a silicon base. Thus the HBT provides improved high-speed and high-frequency operation over the conventional BJT. Increasing the germanium concentration in the silicon-germanium base results in a larger valence band offset between the emitter and the base, leading to enhanced bulk electron and hole mobility, further improving high-speed/high-frequency operation. At a germanium concentration of about 20%, the valence band offset is about 0.17 eV.
Prior art methods for forming an epitaxially grown layer of silicon-germanium overlying a silicon layer (e.g., a silicon-germanium base overlying a silicon collector) carefully control a temperature, a pressure and a reactive gas flow rate during epitaxial growth to achieve germanium concentrations of about 10% to 25% (i.e., about 90% to 75% silicon) in the silicon-germanium layer. As the germanium concentration increases, compressive strain in the silicon-germanium layer increases. Crystalline dislocations form to relieve the strain. The number of dislocations increases as the germanium concentration increases, eventually reaching a level where the dislocations disrupt the epitaxial properties of the silicon-germanium layer, negating the advantageous properties of the silicon-germanium layer. Thus the germanium concentration must be limited to limit the number of dislocations.
Use of a buffer layer (wherein the germanium concentration is varied gradually, with the germanium concentration increasing in a direction away from the collector) between the silicon collector and the silicon-germanium base somewhat reduces strain relaxation and may thereby aid in achieving these concentration levels.
It is known that crystalline defects in a transistor can limit performance. In particular, base region defects, such as the dislocations described above, can reduce the transistor cut-off frequency, current gain and maximum oscillating frequency.